Uncontrolled microbial growth and activity can create severe environmental and human safety problems in wastewater treatment and handling systems associated with municipal, industrial and oilfield operations. Problems caused or intensified by microbial growth and activity include corrosion, solids production, and hydrogen sulfide (H2S) generation. Hydrogen sulfide not only has a highly offensive odor, but is toxic even in very small concentrations.
The microorganisms primarily responsible for H2S generation in an anaerobic environment are sulfate-reducing bacteria. These organisms are ubiquitous and can grow in almost any environment. They are routinely found in waters associated with oil production systems and can be found in virtually all industrial aqueous processes, including cooling water systems, pulp and paper-making systems, chemical manufacturing, and petroleum refining.
Hydrogen sulfide is corrosive and reacts with metal surfaces to form insoluble iron sulfide corrosion products. In oilfield operations, H2S partitions into the water, oil and natural gas phases of produced fluids and creates a number of problems. For instance, oil and gas, which contain high levels of H2S, have a lower commercial value than low sulfide oil and gas. Removing biogenic H2S from sour oil and gas increases the cost of these products. Hydrogen sulfide is an extremely toxic gas and can be lethal to humans at even small concentrations, and poses a threat to worker safety. In addition, the discharge of produced waters containing high levels of H2S into aquatic or marine environments lowers the dissolved oxygen levels in the water as H2S reacts with oxygen.
Corrosion caused by sulfate-reducing bacteria and H2S frequently results in extensive damage. Pipe systems, tank bottoms and other pieces of equipment can rapidly fail if they have areas where microbial corrosion is occurring. If a failure occurs in a pipeline or storage tank bottom, the released fluid can have serious environmental consequences. If a failure occurs in a high pressure water or gas line, the consequences may be worker injury or death. Any such failure involves substantial repair or replacement costs.
Conditions in an oil reservoir subject to seawater flooding are excellent for establishing sulfate-reducing bacteria activity. Conditions within industrial water systems, such as effluent streams from production operations or cooling water streams, are also conducive to sulfate-reducing bacteria activity due to the anaerobic biofilm which is formed on pipeline, tank or vessel walls. The same is true within the sewers and other piping and to facilities associated with municipal wastewater handling systems.
Tanks used to store produced water on floating production, storage, and offloading units (FPSO) are extremely susceptible to generation of high hydrogen sulfide levels because of the activity of sulfate-reducing bacteria. FSPOs and floating storage and offloading units (FSO) are ships containing multiple tanks for separating oil and water, storing oil before offloading into tankers, and processing waters. Produced water typically flows into slop tanks, where it may also combine with drainage water from decks or ballast water from cargo ships. Slop tanks are in many cases the final separation stage in which residual entrained oil is removed from the water before its discharge to the sea, offloading, or re-injection. Environmental conditions in the slop-water tanks, especially the presence of sludge and solids deposits at the bottom of the tanks, are quite favorable for sulfate-reducing bacteria to form biofilms. These solids are also protective to the bacteria and impede the action of chemical biocide treatments for controlling bioactivity. Health, safety, and environmental aspects associated with the presence of the toxic gas on offshore structures make it necessary to implement effective sulfate-reducing bacteria- and H2S-control procedures while still maintaining compliant water for discharge.